1. Field of the Invention
The present invention relates to a honeycomb structure which is suitably used as a dust collecting filter such as a diesel particulate filter.
2. Description of the Related Art
A honeycomb structure has broadly been used as a collecting filter for an exhaust gas such as a diesel particulate filter (DPF) for collecting and removing a particle material (a particulate matter (PM)) such as soot included in an exhaust gas from a diesel engine or the like.
To use such a honeycomb structure (the filter) continuously for a long time, the filter needs to be periodically subjected to a regeneration treatment. That is, the PM deposited in the filter needs to be burnt and removed in order to decrease a pressure loss increased by the PM deposited in the filter with an elapse of time and to return the performance of the filter to an initial state. During the regeneration of this filter, the PM deposited in the filter burns in order from the inlet side of a fluid (an exhaust gas), and hence a temperature intensely rises in a portion closer to the outlet side of the fluid owing to heat generated before the outlet side and heat of the burning of the PM on the spot. In consequence, there are problems that the temperature rise in the filter easily becomes non-uniform and that a defect such as crack is generated by a heat stress.
To solve the problems, a method is suggested in which the honeycomb structure is constituted of a plurality of segments having a honeycomb shape, and the segments are integrally joined via a joining material made of an elastic material, whereby the heat stress acting on the honeycomb structure is scattered and alleviated (e.g., see JP-A-2000-279729). In consequence, a resistance to thermal shock can be improved to a certain degree. However, in recent years, with the increase of the heat stress accompanying the enlargement of the filter, a sufficient effect cannot easily be obtained only by this method.
To solve the problems, an attempt is made to suppress the temperature rise during the regeneration of the filter by impregnating portions of the segments constituting the honeycomb structure near the end surface of the honeycomb structure on the outlet side thereof with a slurry including particles smaller than the average pore diameter of the segments and performing a heat treatment to densify the portions and increase a heat capacity and a thermal conductivity (e.g., see WO2008-78799).
However, in a conventional technology, when the portions of the segments near the end surface of the honeycomb structure on the outlet side thereof are densified as described above, the whole sections of the segments vertical to the axial direction of cells are subjected to a densifying treatment, and hence a pressure loss becomes excessively high, which causes a practical problem on occasion.